A record number of river herring passed safely through a fish ladder on the Presumpscot River in Westbrook last year.

The Friends of the Presumpscot River, the Conservation Law Foundation and Sappi North American reported Wednesday that more than 50,000 river herring ascended the fishway on the Cumberland Mills dam in 2018. That number is a significant increase over the previous two years. In 2017, just 810 river herring were counted at the fishway. In 2016, the number was slightly more than 10,000, compared to 2,960 herring in 2015.

It is unclear why the population fluctuated so dramatically and why this year was so much higher than in the past, although the increase comes several years after the installation of the fish passage. River herring are considered to be a “keystone” species, meaning that herring play a critical role in maintaining the river’s ecosystem for fish, aquatic life and birds.

“These small fish contribute to the biological diversity along the river,” the three parties said in a press release. “Thus, the progress made by many toward attaining access to greater habitat upriver is key to the restoration of this ‘River of Many Falls.'”

The river herring were among 100,000 fish that came up the river in 2018, including 55 shad during 2018. The shad increase is considered significant because in 2017 no shad were counted, according to Michael Shaughnessy, President of the Friends of the Presumpscot River.

The fish ladder at the Cumberland Mills dam was built in 2013 as a result of a settlement agreement between the three parties. A similar agreement announced in 2016 calls for the removal of the Saccarappa Falls dam just one mile up the river from the Cumberland Mills dam. The parties came to those terms in part to help restore fish populations in the Presumpscot River.

The press release Tuesday stated the plan for Saccarappa Falls has received approvals form the Maine Department of Environmental Protection but is waiting for the green light from the federal government.